Trivalent chromium plating solution and plating method using the same

ABSTRACT

Provided is hard trivalent chromium plating solution having improved covering power. The trivalent chromium plating solution comprises a trivalent chromium compound comprising a compound of formula (1) below; 
       wherein Cr 2 (SO 4 ) n (OH) 6-2n  ( n&lt;3)    (1).

RELATED APPLICATION

This application claims priority from Korean Patent Application No.10-2010-0073495 filed on Jul. 29, 2010 in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a trivalent chromium plating solutionand a plating method using the same, and more particularly, to a hardtrivalent chromium plating solution having improved covering power.

2. Description of the Related Art

Chrome plating is the most common process among various platingprocesses and is a final treatment step. Chrome plating may beclassified into decorative plating for obtaining a beautifully glossymetal surface and hard plating for increasing wear resistance.

In the case of decorative plating, the plating thickness typicallyranges from 0.2 to 0.5 μm. Decorative plating is usually used to platebrass surfaces of tableware and other disposable products. In the caseof hard plating, the plating thickness may range from approximately 5 toseveral tens of μm although it varies according to the required wearlife. Hard plating is used to prevent a portion of a camera lens towhich a screw is tightened, a precision machine, an inner surface of amold, a surface of a printing plate, etc. from wearing out.

In hexavalent chromium plating, electroplating is typically performedusing a solution of chromic acid (CrO₃) mixed with sulfuric acid(H₂SO₄). For an anode, an element that is not eroded by H₂SO₄, such aslead, is used instead of chromium metal. Electrodeposition iscontinuously performed while CrO₃ supplement a reduction of chromium inthe solution.

The concentration of CrO₃ in a plating solution used for chrome platingmay be high or low, and various forms of coatings can be obtaineddepending on the temperature of the solution and current density, thatis, plating conditions. Advantages of hexavalent chromium platinginclude excellent reflectivity, color and corrosion resistance as wellas high current efficiency.

Despite these advantages, hexavalent chromium plating produces CrO₃ gasthat is fatal to the human body during the plating process. Inparticular, hexavalent chromium ions introduced into underground wateror rivers may cause deadly environmental contamination. Therefore,hexavalent chromium must be reduced to trivalent chromium.

That is, hexavalent chromium is classified as a carcinogen by theInternational Agency of Research on Cancer (IARC). Since the use ofhexavalent chromium is expected to be prohibited, alternatives tohexavalent chromium are required. Accordingly, a lot of research isbeing actively conducted worldwide to develop alternatives to hexavalentchromium.

Examples of alternatives to hexavalent chromium plating includeion-nitriding, plasma spraying, and ion plating. However, thesealternatives require 5 to 10 times the cost of hexavalent chromiumplating and cannot be applied to large-sized products.

For these reasons, chrome plating using trivalent chromium is beingrecognized as the most efficient alternative.

However, a conventional trivalent chromium plating solution does nothave superior covering power. Thus, it is difficult to applyelectroplating on surfaces of machine parts or products havingcomplicated shapes.

SUMMARY OF THE INVENTION

Aspects of the present invention provide a trivalent chromium platingsolution having superior covering power.

However, aspects of the present invention are not restricted to the oneset forth herein. The above and other aspects of the present inventionwill become more apparent to one of ordinary skill in the art to whichthe present invention pertains by referencing the detailed descriptionof the present invention given below. According to an aspect of thepresent invention, there is provided a trivalent chromium platingsolution comprising a trivalent chromium compound including a compoundof formula (1) below;

wherein Cr₂(SO₄)_(n)(OH)_(6-2n) (n<3)   (1).

According to another aspect of the present invention, there is provideda plating method comprising preparing a trivalent chromium platingsolution in a plating bath, immersing an object to be plated in thetrivalent chromium plating solution, and applying a negative potentialto the object to be plated and applying a positive potential to aninsoluble anode after installing the insoluble anode in the platingbath, wherein the trivalent chromium plating solution comprises atrivalent chromium compound comprising a compound of formula (1) below,a complexing agent suppressing a polymerization reaction of thetrivalent chromium compound in the plating solution, a conductivityagent increasing electrical conductivity of trivalent chromium ions, abuffer agent stabilizing a hydrogen ion index of the plating solution, aplating activating additive increasing adhesive power and capability offorming a plated layer, a wetting agent removing a pitting phenomenonwhich occurs on a plated surface, and a hydrogen ion index adjustingagent adjusting the hydrogen ion index of the plating solution,

wherein Cr₂(SO₄)_(n)(OH)_(6-2n) (n<3)   (1).

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects and features of the present invention willbecome more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings, in which:

FIG. 1 is a process flowchart illustrating a method of preparing atrivalent chromium solution according to the present invention; and

FIG. 2 is a flowchart illustrating a plating process using a trivalentchromium plating solution according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied indifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

A trivalent chromium plating solution according to the present inventionmay include a trivalent chromium compound which provides trivalentchromium ions, a complexing agent which suppresses a polymerizationreaction of the trivalent chromium compound in the plating solution, aconductivity agent which increases electrical conductivity of thetrivalent chromium ions, a buffer agent which stabilizes a hydrogen ionindex of the plating solution, a plating activating agent whichincreases adhesive power and capability of forming a plated layer, awetting agent which removes a pitting phenomenon that occurs on a platedsurface, and a hydrogen ion index adjusting agent which adjusts thehydrogen ion index of the plating solution.

The trivalent chromium compound includes a compound of formula (1)below.

Cr₂(SO₄)_(n)(OH)_(6-2n) (n<3)   (1).

The trivalent chromium compound may be added to the trivalent chromiumplating solution in an amount of 0.4 mol/L to 1.3 mol/L, particularly,0.8 mol/L to 1 mol/L. Trivalent chromium contained in the trivalentchromium compound forms a chromium film.

The complexing agent may prevent the trivalent chromium ions containedin the trivalent chromium compound from being polymerized in the platingsolution. The complexing agent may be, for example, formic acid, alkalimetal formate, or ammonium formate, but is not limited thereto. Thecomplexing agent may be added to the trivalent chromium plating agent inan amount of 0.05 to 2 mol/L, particularly, 0.4 to 0.6 mol/L.

The conductivity agent may facilitate the formation of a chromium filmby increasing electrical conductivity when an object to be plated isimmersed in the trivalent chromium plating solution so as to be plated.The conductivity agent may be, for exampe, sodium sulfate, but is notlimited thereto. The conductivity agent may be added to the trivalentchromium plating solution in an amount of 0.1 mol/L to 0.8 mol/L,particularly, 0.2 mol/L to 0.4 mol/L.

The buffer agent enables the hydrogen ion index (pH) of the trivalentchromium plating solution to remain stably within a predetermined range.The buffer agent includes boric acid and aluminium sulfate. As thebuffer agent, when both boric acid and aluminium sulfate exist in thetrivalent chromium plating solution, high-quality plating can beobtained. Boric acid may be contained in the trivalent chromium platingsolution in an amount of 0.08 to 1 mol/L, particularly, 0.4 to 0.6mol/L. Aluminium sulfate may be contained in the trivalent chromiumplating solution in an amount of 0.05 to 0.2 mol/L, particularly, 0.1 to0.16 mol/L.

The plating activating agent facilitates plating using low-current,thereby expanding an effective plating current range. The platingactivating additive may be, for example, carbamide, but is not limitedthereto. The plating activating agent may be contained in the trivalentchromium plating solution in an amount of 0.1 to 1.2 mol/L,particularly, 0.4 to 0.6 mol/L.

The wetting agent may remove the pitting phenomenon that occurs on aplated surface. The wetting agent may be a surfactant. An example of thesurfactant is an organic anion-active surfactant containing a sulfonategroup, but is not limited thereto. An example of the organicanion-active surfactant is sodium lauryl sulfate (C₁₂H₂₅SO₄Na). Thewetting agent may be contained in the trivalent chromium platingsolution in an amount of 0.01 to 1 g/L, particularly, 0.05 to 0.1 g/L.

The hydrogen ion index adjusting agent may adjust the hydrogen ion indexof the trivalent chromium plating solution. The hydrogen ion indexadjusting agent may be, for example, sulfuric acid (H₂SO₄), sodiumhydroxide (NaOH), or sodium carbonate (Na₂CO₃), but is not limitedthereto. The hydrogen ion index adjusting agent may be added to thetrivalent chromium plating solution in an amount that allows thehydrogen ion index of the trivalent chromium plating solution to be in arange of, e.g., 1.1 to 3.5, particularly, 1.4 to 1.8.

Hereinafter, a method of preparing a trivalent chromium plating solutionaccording to the present invention will be described with reference toFIG. 1. FIG. 1 is a process flowchart illustrating a method of preparinga trivalent chromium solution according to the present invention.

Referring to FIG. 1, a trivalent chromium compound is added to heated,distilled water and is then stirred (operation S100). Here, thetemperature of the distilled water may be approximately 70 to 95° C.,and the trivalent chromium compound is stirred until it is completelydissolved in the distilled water.

The solution having the trivalent chromium compound dissolved therein isfiltered (operation S110). The solution may be filtered using a paperfilter, a fiber filter, or other types of filters.

A buffer agent and a conductivity agent are added and dissolved in thefiltered solution (operation S120). Here, the temperature of thesolution may be 70 to 80° C., and the added buffer agent andconductivity agent may be solid. The above solid elements should becompletely dissolved in the solution.

Next, a complexing agent is added to the solution in which the bufferagent and the conductivity agent have been dissolved (operation S130).The solution having the buffer agent and the conductivity agentdissolved therein is maintained at a temperature of 70 to 80° C., andthe complexing agent is slowly added to the solution.

A plating bath is covered with a covering member, and the solutionhaving the complexing agent added thereto is maintained at 70 to 80° C.for a predetermined period of time, for example, for approximately onehour (operation S140).

Next, the solution maintained at 70 to 80° C. for the predeterminedperiod of time is cooled (operation S150). The solution may be cooled toapproximately 40 to 50° C.

A plating activating agent is added and dissolved in the cooled solution(operation S160). The plating activation agent may be solid and iscompletely dissolved in the solution at a temperature of 40 to 50° C.

Then, the plating bath is covered with the covering member, and thesolution having the plating activating agent dissolved therein ismaintained at a temperature of 40 to 50° C. for a predetermined periodof time, for example, for approximately one hour (operation S170).

The solution maintained at 40 to 50° C. for the predetermined period oftime is cooled (operation S180). The solution may be cooled to roomtemperature.

The volume of a plating solution is adjusted by adding distilled waterto the cooled solution. In addition, a hydrogen ion index of thesolution is measured, and, if necessary, the hydrogen ion index of thesolution is adjusted by adding a hydrogen ion index adjusting agent tothe solution (operation S190).

Next, a wetting agent is added to the solution having the adjustedhydrogen ion index (operation S200). Specifically, the wetting agent maybe added in a predetermined concentration to distilled water, and thedistilled water having the wetting agent may be added to the solutionhaving the adjusted hydrogen ion index.

Distilled water is added to the solution having the wetting agent inorder to reach the final volume of the plating solution. Then, thehydrogen ion index of the solution is measured, and, if necessary, thehydrogen ion index of the solution is adjusted again by adding thehydrogen ion index adjusting agent (operation S210).

Hereinafter, a plating process using a trivalent chromium solutionaccording to the present invention will be described with reference toFIG. 2. FIG. 2 is a flowchart illustrating a plating process using atrivalent chromium plating solution according to the present invention.

Referring to FIG. 2, a trivalent chromium plating solution according tothe present invention is prepared within a plating bath (operationS300). Then, an object to be plated is immersed in the trivalentchromium plating solution (operation S310).

A negative potential is applied to the object to be plated. In addition,an insoluble anode is installed in the plating bath, and a positivepotential is applied to the insoluble anode (operation S320). Here, theinsoluble anode may be, for example, a titanium-manganese dioxide anode(TMDA), a dimensionally stable anode (DSA) having iridium oxide (IrO₂)or ruthenium oxide (RuO₂) formed on a porous titanium plate, or aplatinized titanium anode, but is not limited thereto. In particular,TMDA does not require spatial separation of a cathode and an anode in aplating process and does not release toxic chlorine gas during theplating process. Furthermore, TMDA noticeably reduces an electrochemicaloxidation reaction from trivalent chromium to hexavalent chromium.

When power is supplied, the current density may be 15 to 30 A/dm²,particularly, 10 to 20 A/dm².

During the plating process, a hydrogen ion index of the plating solutionmay be maintained at 1.1 to 3.5, particularly, 1.4 to 1.8. In addition,the plating bath may be maintained at a temperature of 20 to 60° C.,particularly, 30 to 40° C.

A chromium-plated film may have a thickness of several tens of μm.

Hereinafter, the present invention will be described in greater detailby way of specific embodiments and a comparative example.

Embodiment 1

To prepare 1 L of plating solution, 1 mol of the compound of formula (1)was added to 0.5 to 0.7 L of heated, distilled water and was stirred at70 to 95° C. until it was completely dissolved.

Cr₂(SO₄)_(n)(OH)_(6-2n) (n<3)   (1).

Then, the solution having the compound of formula (1) added thereto wasfiltered, and 0.15 mol of aluminium sulfate, 0.5 mol of boric acid, and0.3 mol of sodium sulfate were added to the filtered solution at 70 to80° C. These added solid elements were completely dissolved. Next, 0.5mol of formic acid was slowly added to the solution which containedaluminium sulfate, boric acid and sodium sulfate and which wasmaintained at 70 to 80° C. Then, a plating bath that contained thesolution having formic acid added thereto was covered with a coveringmember and was maintained at 70 to 80° C. for approximately one hour.The solution maintained at 70 to 80° C. for approximately one hour wascooled to 40 to 50° C. After the cooling of the solution, 0.5 mol ofsolid carbamide was added to the solution being stirred at 40 to 50° C.and was completely dissolved. The plating bath that contained carbamidewas covered with the covering member and was maintained at 40 to 50° C.for approximately one hour. Then, the solution maintained at 70 to 80°C. for approximately one hour was cooled to room temperature, anddistilled water was added to the cooled solution such that the volume ofthe plating solution became 0.9 to 0.95 L. Thereafter, a hydrogen ionindex of the solution having the distilled water added thereto wasmeasured and, if necessary, was adjusted to 1.5 by adding a H₂SO₄, NaOH,or Na₂CO₃ solution to the solution. Using a sodium laurylsulfate(C₁₂H₂₅SO₄Na) solution prepared in a concentration of 10 g/L inadvance, 0.1 g of C₁₂H₂₅SO₄Na was added to the solution having theadjusted hydrogen ion index. After the addition of C₁₂H₂₅SO₄Na, thevolume of the plating solution was adjusted to a final volume of 1 Lusing distilled water. Then, the hydrogen ion index of the solutionwhose volume had been adjusted to the final volume was measured againand, if necessary, was adjusted to 1.5 by adding the H₂SO₄, NaOH, orNa₂CO₃.

Embodiment 2

A plating solution was prepared under the same conditions as Embodiment1, except that the hydrogen ion index was adjusted to 1.7.

Embodiment 3

A plating solution was prepared under the same conditions as Embodiment1, except that the compound of formula (1) was added in an amount of 0.8mol.

Embodiment 4

A plating solution was prepared under the same conditions as Embodiment1, except that sodium sulfate was added in an amount of 0.2 mol.

Embodiment 5

A plating solution was prepared under the same conditions as Embodiment1, except that the compound of formula (1) was added in an amount of 0.8mol and that formic acid was added in an amount of 0.4 mol.

Embodiment 6

A plating solution was prepared under the same conditions as Embodiment1, except that carbamide was added in an amount of 0.4 mol and that thehydrogen ion index was adjusted to 1.4.

COMPARATIVE EXAMPLE 1

A plating solution was prepared by adding 0.5 mol/L of chromium sulfateinstead of the compound of formula (1), 0.18 mol/L of aluminium sulfate,0.63 mol/L of sodium sulfate, 0.45 mol/L of carbamide and 0.66 mol/L ofsodium formate. In addition, the hydrogen ion index was adjusted to 1.4.

The covering power of each of the plating solutions of Embodiments 1through 6 and Comparative Example 1 was evaluated. Specifically, a discelectrode (S=1.77 cm²) of a copper foil fixed to a plastic holder wasused. Electroplating was conducted using TDMA at a constant currentdensity and without the spatial separation of a cathode and an anode.The volume of the plating solution was 0.5 L. The current density wasgradually increased by each 1 A/dm². Electroplating was conducted for 30seconds for all current density values. A minimum current density whenchromium was plated on the whole surface of the cathode was visuallydetermined, and the covering power of the plating solution was measuredbased on the determination result. The measurement results are shown inTable 1. During the plating process, the temperature of a plating bathwas 35° C.

TABLE 1 Minimum current density value (A/dm²) that allows a chromiumcoating to be electro- deposited 100% on a surface of a cathodeComparative Example 1 22 Embodiment 1 15 Embodiment 2 14 Embodiment 3 17Embodiment 4 19 Embodiment 5 17 Embodiment 6 18

As apparent from the results shown in Table 1, the minimum currentdensities of the plating solutions of Embodiments 1 through 6 are farlower than that of the plating solution of Comparative Example 1.Accordingly, the plating solutions of Embodiments 1 through 6 using thetrivalent chromium compound of formula (1) have better covering powerthan the plating solution of Comparative Example 1 using a trivalentchromium compound such as chromium sulfate.

A trivalent chromium plating solution according to the present inventionhas superior covering power. In addition, a compound of formula (1),which can also be used as a tanning agent, can be mass-produced and isinexpensive. Therefore, the trivalent chromium plating solution is thefirst-ever plating solution that provides the compound of formula (1) asa trivalent chromium compound. Using the compound of formula (1) as asource, an economical trivalent chromium plating solution can beproduced.

In concluding the detailed description, those skilled in the art willappreciate that many variations and modifications can be made to thepreferred embodiments without substantially departing from theprinciples of the present invention. Therefore, the disclosed preferredembodiments of the invention are used in a generic and descriptive senseonly and not for purposes of limitation.

1. A trivalent chromium plating solution comprising: a trivalentchromium compound comprising a compound of formula (1) below;wherein Cr₂(SO₄)_(n)(OH)_(6-2n) (n<3)   (1).
 2. The plating solution ofclaim 1, wherein the plating solution further comprising: a complexingagent suppressing a polymerization reaction of the trivalent chromiumcompound in the plating solution; a conductivity agent increasingelectrical conductivity of trivalent chromium ions; a buffer agentstabilizing a hydrogen ion index of the plating solution; a platingactivating agent increasing adhesive power and capability of forming aplated layer; a wetting agent removing a pitting phenomenon which occurson a plated surface; and a hydrogen ion index adjusting agent adjustingthe hydrogen ion index of the plating solution,
 3. The plating solutionof claim 1, wherein the trivalent chromium compound is added in anamount of 0.4 to 1.3 mol/L.
 4. The plating solution of claim 2, whereinthe complexing agent is at least one selected from the group consistedof a formic acid, alkali metal formate and ammonium formate and is addedin an amount of 0.05 to 2 mol/L.
 5. The plating solution of claim 3,wherein the conductivity agent is sodium sulfate and is added in anamount of 0.1 to 0.8 mol/L.
 6. The plating solution of claim 4, whereinthe buffer agent comprises boric acid and aluminium sulfate, wherein theboric acid is added in an amount of 0.08 to 1 mol/L, and the aluminiumsulfate is added in an amount of 0.05 to 0.2 mol/L.
 7. The platingsolution of claim 5, wherein the plating activating agent is carbamideand is added in an amount of 0.1 to 1.2 mol/L.
 8. The plating solutionof claim 6, wherein the wetting agent is sodium lauryl sulfate and isadded in an amount of 0.01 to 1 g/L.
 9. The plating solution of claim 7,wherein the hydrogen ion index adjusting agent may be at least oneselected from the group consisted of a sulfuric acid (H₂SO₄), sodiumhydroxide (NaOH), and sodium carbonate (Na₂CO₃).
 10. The platingsolution of claim 1, having a hydrogen ion index of 1.1 to 3.5.
 11. Aplating method comprising: preparing a trivalent chromium platingsolution in a plating bath; immersing an object to be plated in thetrivalent chromium plating solution; and applying a negative potentialto the object to be plated and applying a positive potential to aninsoluble anode after installing the insoluble anode in the platingbath, wherein the trivalent chromium plating solution comprises: atrivalent chromium compound comprising a compound of formula (1) below;a complexing agent suppressing a polymerization reaction of thetrivalent chromium compound in the plating solution; a conductivityagent increasing electrical conductivity of trivalent chromium ions; abuffer agent stabilizing a hydrogen ion index of the plating solution; aplating activating additive increasing adhesive power and capability offorming a plated layer; a wetting agent removing a pitting phenomenonwhich occurs on a plated surface; and a hydrogen ion index adjustingagent adjusting the hydrogen ion index of the plating solution,wherein Cr₂(SO₄)_(n)(OH)_(6-2n) (n<3)   (1).
 12. The plating method ofclaim 11, wherein the insoluble anode is a titanium-manganese dioxideanode (TMDA), a dimensionally stable anode (DSA) having iridium oxide(IrO₂) or ruthenium oxide (RuO₂) formed on a porous titanium plate, or aplatinized titanium anode.
 13. The plating method of claim 11, whereinin the applying of the negative potential and the applying of thepositive potential, a current density is 15 to 30 A/dm².
 14. The platingmethod of claim 11, wherein the plating bath is maintained at atemperature of 20 to 60° C.